My invention pertains to a protective electrical discharge device. More specifically, the invention relates to a lightning arrester device for protecting low voltage circuits and/or apparatus against voltage surges, particularly from lightning during electrical storms.
Many low voltage signaling and communication circuit arrangements, and railroad signaling circuits and apparatus in particular, are frequently exposed to hazards such as high voltage surges on input connections resulting from electrical storms. Therefore, to maintain proper and safe operation, these circuits require adequate protection against such high voltage from lightning and other external causes. Protective discharge devices, e.g., lightning arresters, have been used for many years to protect low voltage circuits and apparatus. Various types and materials have been incorporated in designs for both line-to-line and line-to-ground protection paths. The advantages and benefits of using varistors, i.e., non-linear resistors, in surge protection devices have long been recognized. Specifically, in the construction of air gap lightning arresters, the use of a varistor material for one of the pair of arc contacts gives superior performance as compared to metal-to-metal arc contacts, since the grain structure of the varistor facilitates ionization initiation of the air gap. This results in faster ionization and within a narrower potential range than is normally attainable with a metal-to-metal air gap. A major problem in this type of device is the relatively low energy capacity of varistor material which leads to varistor destruction by high surge currents. Thus a design arrangement for a protective electrical discharge device whereby efficient yet economic varistor elements may be used, with destruction of the varistor when a high voltage discharge occurs, is highly desirable.
Accordingly, an object of my invention is an improved protective electrical discharge device incorporating a varistor element.
Another object of the invention is a high voltage protective discharge device using a varistor element as an initial arc contact.
A further object of the invention is an improved lightning arrester employing a varistor-to-metal air gap to initiate discharge with a parallel, back-up metal-to-metal air gap to dissipate the high voltage energy surge to protect the varistor element.
Yet another object of the invention is a lightning arrester including a first air gap between a varistor element and a metal electrode separated by a non-conducting washer, across which an initial surge discharge occurs, and a parallel air gap between that electrode and a second metal electrode, in contact with the varistor, to provide a heavy duty energy surge discharge path as the voltage increases.
Other objects, features, and advantages of the invention will become apparent from the following specification and appended claims, when taken in connection with the accompanying drawings.